Frequency shift carrier system



March 1, 1949. G. J. MAKI FREQUENCY SHIFT CARRIER SYSTEM Filed July 50, 1946 GEORGE 1 MA Kl INVENTOR.

Patented Mar. 1, 1949 2,463,402 FREQUENCY SHIFT CARRIER SYSTEM George J. Maki, Cedar Rapids, Iowa,

Collins Radio Compan a corporation of Iowa Application July 30, 194 6, Serial No. 687,056

9 Claims.

This invention relates to signal transmission and receiving systems and more particularly to systems such for example as telegraph systems, wherein different signal conditions are represented respectively by frequencies.

It has been proposed heretofore to transmit different signals, such for example as telegraph mark and space, through the intermediary of two different frequencies, one of which represents mark and the other of which represents space. This has usually been accomplished either by employing two separate frequency sources at the transmitter which are alternately keyed into a circuit in accordance with the mark and space conditions, or a single frequency such as a carrier or subcarrier which is frequencyshifted between frequency limits representing mark and space. At the receiving end of the prior systems, it has been the practice to detect the two frequencies and to convert them into local telegraph signals through the intermediary of conventional discriminators to produce two different frequencies or tones which after rectification are applied to a responder such as a differential or polar relay. One of the serious difilculties in these prior receiving arrangements is the relatively low margin of discrimination between the two signals applied to the responder, it being necessary to employ expensive band-pass filters for the respective mark and space signals.

Accordingly, it is a principal object of this invention to provide a telegraph receiving arrangement for receiving so-called lei-frequency signals wherein the margin of discrimination in the signals actually applied to the telegraph responder is substantially increased.

Another principal object is to provide a receiving arrangement for bi-frequency telegraphy, wherein the ratio of useful signal-to-noise is greatly increased by employing an improved frequency discriminator and tone keyer arrangement.

Another principal object is to increase the accuracy and reliability of signal reproduction in bi-frequency telegraph systems and the like.

A feature of the invention relates to a bi-frequency telegraph converter arrangement for converting received bi-frequency signals representing mark and space into mark and space signals of the same frequency or tone, the separation of the mark and space signals being achieved in such a way as to reduce the likelihood of crosstalk between the mark and space channels.

i Another feature relates to a bi-frequency telegraph converter arrangement employing a composite frequency discriminator and frequency converter unit in the form of a pair of grid-controlled tubes which are excited by the detected bi-frequency signals and by a local source of oscillations at the mean frequency between the assignor to y, Cedar Rapids, Iowa,

said detected bi-frequency signals. As a result, there are produced mark and space signals of the same frequency but in respective channels and with a minimumof cross-talk therebetween.

A further feature relates to a novel frequency discriminator arrangement employing a pair of multi-grid electron tube detectors preferably, although not necessarily, of the pentode type which are normally biased near plate current cutoff.

A further feature relates to a novel tone keyer arrangement employing a pair of pentodes whose control grids are excited in phase opposition under control of received detected frequencies and whose shield grids are used as injection grids excited in phase by a local oscillator of a frequency which is the mean between the frequencies of the detected frequencies. As a result of this arrangement, it is possible to connect two separate channels e. g., telegraph mark and telegraph space channels respectively to the plate circuits of said pentodes with a maximum of on and off keying discrimination between the two channels. Furthermore, since the pentodes pass plate current only when their control grids are excited by respective mark or space signals, there is a considerable reduction in the loading of the output Winding of the discriminator thus providing sharper discriminator characteristics.

A still further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to provide an improved telegraph receiving system.

Other features and advantages not particularly enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims.

In the drawing,

Fig. 1 is a schematic block and wiring diagram of a typical telegraph system embodying features of the invention.

Figs. 2 and 3 are graphs explanatory of the operation of Fig. 1.

While the invention will be described herein as embodied in a telegraph system, it will be understood that this is done merely for explanatory purposes. The inventive concept is equally well applicable to other systems of signalling e. g., facsimile, or any system where signal indications are to be produced at a distance representing for example measurements of values, pressures, voltages, etc., where different conditions to be measured are represented by different frequencies.

The invention is particularly well suited for use in converting bi-frequency telegraph signals, such for example as Well-known telegraph frequency shift signals, into a suitable form for the operation of Morse equipment, printing telegraph or teletype equipment, or indeed any responder which is controlled by coded impulses of the Baudot type. For simplicity of explanation, the

matically by block term bi-frequency as employed herein means that one transmitted and detected frequency represents a telegraph mark condition, and another transmitted frequency represents a telegraph space condition. These two frequencies are transmitted alternately in the usual manner so. that either one or the other is always present. Preferably, although not necessarily, the bi-frequency signals are relatively close in the. frequency spectrum, the absolute value of their frequency difference being small compared to either frequency. For example, in the audible frequency range, a possible choice would be 1050 C. P. S. Likewise, in the radio frequency range, the frequencies could be 1000 k. c. and

Referring to Fig. 1, there is represented schel any well-known form of signal transmitter for generating and transmitting the bi-frequency signals. Merely for explanation, it will be assumed that transmitter I0 is a radio transmitter which transmits two radio frequency carriers of 1000 k. c. and 1000.5 k. c. representing respectively mark and space, the transmission channel ll being of any well-known type. The radio signals are received in any suitable radioreceiver preferably, although not necessarily, of the superheterodyne type, the radio frequency and frequency conversion stages of which are represented by blocks I2 and I3. In the event that the frequency signals hadbeen transmitted over a Wire or physical channel, then the conductors l4 would represent the incoming terminals of such a channel. With the example above given, the intermediate frequency signals at the output of stage l3- may have a frequency ofv 100 k. c. and 100.5 k. c; respectively. These intermediate frequencies are then coupled by intermediate frequency transformer l5 to a current-limiting amplifier l6 preferably of the-pentode type or of the so-called beam power type, or of any other type whose output or plate current is rendered substantially independent, Within certain practical limits, of the input signal voltages. Any well-known current or peak limiter such as ordinarily used in frequency modulation radio receivers can be employed. The secondary of transformer is tuned to the intermediate frequencies and is connected across the control grid I! and cathode IS in series with resistor l9 and its shunt condenser 20. The suppressor grid 2! is directly connected to cathode l8 and the shield grid 22 is connected to the positive terminal 23 of the D. C. plate power supply through its current-limiting resistor 24. Resistor 24 is shunted to ground through condenser 25 in the conventional manner.

The frequency discriminator network comprises a pair of mutually coupled inductances 26, 21, which are tuned by respective condensers 28, 29, to the mean frequency F between the mark and space frequencies. In the above example, the frequency F would then be 100.25 k. c. The relation between this mean frequency and the mark and space frequencies is illustrated in the graphs of Figs. 2 and 3. The center point of secondary 21 is returned to ground through a high resistance to provide a D. C. return path for the control grids 3i, 32, of detector-modulator tubes 33, 34. Capacitor also provides electrostatic coupling from the anode of tube 16, to the grids 3| and 32. The mutual inductance between inductances 2G and 2'! causes grids 3|, 32, to be excited in phase opposition; while the coupling through 950. and

ation for the capacitor 35 causes these grids to be excited in likephase.

When the windings 26 and 21, are tuned to the said mean or center frequency F, their mutual inductance can be adjusted so that the vector sum of the electrostatic coupling and the electromagnetic coupling causes excitation voltages to appear respectively atv grids 3i and 32 (with respect to their common cathode connection 35) for any given value of frequency other than F, to be substantially different from each other. This is true provided of course that the signal frequencies impressed upon the discriminator network do not depart from the mean frequency F by an amount exceeding; the capabilities of the transformer 26, 2'1, to exhibit this frequency discriminating phenomenon. For purposes of explanation, F can be considered the center or mean frequency, and FiAF will represent the applied frequencies above and below F by some finite value AF.

It is an important feature of the present invention that the ratioof anode-cathode space current of tubes 33 and 34 be as high as possible for applied voltages FiAF. In accordance with this phase, of the invention, the tubes 33 and 3d are arranged. to act as non-linear or power law detectorsso that the ratio of their respective output currents Willbe equal to the ratio of their respectiveapplied grid voltages raised to some exponential power. This is in contra-distinction to the conventional diode type detectors which. follow an. essentially linear law. Thus, when frequency F-i-AF is applied, tube 33 receives more excitation at its control grid. 3! than does the control grid 32 of tube 34 because of their respective vector relationships. The ratio of the excitation voltages thus applied to grids 3 l. and 32 as a result of the frequency discriminating action, should be large so as to provide the proper operapparatusor telegraph responder 31.. For example, if tubes 33 and 34 are adjusted for square law detection and the ratio of the control grid voltages applied, to their respective control grids is five, then the ratio of output voltages will be approximately twenty-five.

In order further to accentuate the high ratio of outputs from the tubes 33 and 34, the D.. C. plate supply source 38 is connected across three voltage divider resistances 39, 40, M, thus, providing additional bias whereby the tubes 33. and 34- can be biased to; cutofi except when they are passing plate current corresponding to a respective. impressed mark or space signal. The voltage drop across resistor 4|. is equal to the anodecathode space currents of tubes 33 and 34 plus the current through resistor 40 times the resistance of. resistor 4|. In other words, E4l-=R4l(I33+I-34I-I40). Resistances 40 and Al can be adjusted so as to. produce sufficient bias to, cut off anode-to-cathod'e space current of tubes. 33 and 34, when no grid excitation is applied. Thus the ratio of the anode-to-cathode space current of. the tube receiving the greater grid excitation, to the anode-cathode space current of the tube receiving the lesser grid excitation, may be madev to approach infinity.

In addition, to acting as square. law detectors, the tubes 33, and 34 act as keying modulators. For this purpose, there is provided; a local oscillater 42 which may generate a tone or alternating current in the audio-frequency range and which can be connected into circuit by switch SI. These. tone currents are injected by means of the coupling transformer 43 in like phase into the screening grids 4'4, 45. Screen grids 44 and 45 are polarized to the necessary positive D. C. potential through resistor 39 connected in series with the secondary winding 46 of the transformer 43. In the absence of any injected frequency from source 42, the output of the discriminator would be a direct current at either of the plate circuits of tubes 33, 34. The injection of the audio-frequency tone ,f modulates this direct current at the same frequency f. The amplitude of this audio-frequency output tone f is therefore a function of-the grid excitation voltage of the respective tube 33 or 34, and the injection voltage from source 42. That is to say, the audio-frequency tone I will be greater in that particular output transformer which is associated with the tube receiving the greater grid excitation. n the other hand, because of the cutofi bias provided by resistors 4| and source 38, the space current is zero for the other tube which receives the lesser grid excitation. The net result is that the audio-frequency tone will also be zero in the output of this particular tube. Transformers 4? and 48, as are the subsequent units 49, 50 and 5|, 52, are purposely designed so as to pass only the frequency f and its useful components or side bands. The units 49 and 50 represent suitable low-pass filters for the audio-frequency tone current i. These currents can then be applied to respective amplifier rectifier units 5|, 52, of conventional design. The rectifier units are connected in the usual way to a transducer 31, which may take the form of a printing telegraph receiver, facsimile receiver, Morse sounder or undulat-or or the like. It will be understood that the transducer 31 is provided with a suitable differential receiving means which follows the respective mark and space alternations of the signalling code. For example, if a polar or differential direct current relay is connected to the devices 5| and 52 in the usual manner, plate current from one of the tubes, for example tube 33, will cause the relay contacts to close in one direction representing for example a mark signal; and a current in the output of the other tube, for example tube 34, will cause the contacts of the said relay to close in the opposite direction representing for example a space signal.

While in the foregoing, it has been mentioned that the tubes 33 and 34 are preferably of the pentode detector type, it will be understood that the invention is not necessarily limited to this type of tube.

Since the detectors 33 and 34 present a nearly infinite impedance to the winding 21 of the discriminator transformer, this substantially eliminates the loading elfect on the said transformer enabling a higher selectivity to be realized in the discriminator. This increased selectivity manifests itself in a steeper discriminator slope. This increased selectivity in the discriminator also aids in the rejection of undesired voltages such as interference and noise voltages which might impair reception and utilization of the signals.

While the drawing shows the local injection oscillator 42 coupled to the tubes 33 and 34 through the intermediary of a coupling transformer, it will be understood that any other method of connecting the said source to the grids 44 and 45 may be employed.

It will also be understood that the invention is not limited to the use of an injection source 42 of audio frequency. This injection source may be of any suitable super-audible or even radio frequency F which can be connected into circuit by a corresponding switch S2 so as to replace the source 1. The frequency F will then be the same as the mean frequency between the limits of the frequency-shifted signals applied to the grids 3|, 32. As a result of the de-modulating action in the tubes 33 and 34, there is produced in the outputs 41 and 48 a frequency AF representing the numerical difierence between the frequency of source F and the applied frequency-shifted signals. In this case both filters 49 and 50 can be identical in design.

One of the important advantages of the above described system, apart from the sharpness of discrimination between the mark and space signals, is the fact that since the transducer 31 is controlled by signals of the same frequency, namely i in the case of audio-frequency injection or AF in the case of super-audible or radio frequency injection, the cost of the filter equipment represented by elements 49 and 50 is merely reduced since it is not necessary to design separate frequency filters as is the case with conventional arrangements, wherein the transducer 31 is controlled by two different frequencies. Furthermore, in the case of audio frequency injection it is possible to operate the system over much wider limits of value of frequency shift as compared to existing systems since the control signals from the tubes 33 and 34 which represent mark and space respectively are of the same frequency. It is not necessary therefore to change filters if a different change in shift values is employed for transmission.

Another important advantage of the described system is that the tubes 33 and 34 act in the nature of constant current sources for the differential relay in the transducer 31. As above described, the plate currents of tubes 33, 34 are used to bias them to plate current cutoff. Consequently, signal amplitude variations above a certain minimum from the limiter tube l6 have no further effect upon the differential relay current. In conventional systems, Where two amplifier tubes are used to control the differential relay current, it is the practice to unbias the D. C. amplifiers in order to operate the differential relay. The relay current is therefore a function of the grid voltage. The net result in such prior systems is the possibility of distortion if the differential relay currents become unbalanced. Since the mark and space signals at the transformers 41 and 48 are substantially independent of amplitude variations in the received signals applied to the input of tube l6, and because of the current-limiting action of this tube IS, a large proportion of noise components is eliminated. Furthermore, because of this noise peak clipping, the band width of the original pulse is reduced by a factor proportional to the amplitude ratio before and after clipping, rather than to the area or power ratio. This also tends to improve the overall signal-to-noise ratio.

Various changes and modifications may be made in the disclosed embodiment without departing from the spirit and scope of the invention.

What is claimed is:

1. A receiving arrangement controlled by a plurality of frequencies, comprising a frequency discriminator network, a local source of sustained electrical oscillations, a modulator arrangement excited simultaneously by the signals from the discriminator and by the oscillations from said local source, a pair of separate channels connected to the output of the modulator and having filter means to select the same modulation frequency produced in each channel, means normally biasing said modulator to plate current cutoff so that for one received frequency one of said channels is keyed on while the other is keyed off, and a signal transducer connected to both said channels.

2. A receiving arrangement controlled by frequency shift signals F-l-AF and F-AF, comprising a frequency discriminator network, means to tune said network to the frequency F representing the mean between the limits of the frequency shift signals, a pair of grid-controlled modulator tubes biassed to'act as detectors of the squarelaw type, a local source of injection frequency, means to excite said modulator by the signals from said discriminator and by the current from said local source to produce a beat frequency, a pair of separate channels connected respectively to the outputs of said tubes each of said channels being selective to only the same audio frequency, and a signal transducer connected in common to both said channels, said grid-controlled tubes being normally biassed to plate current cutoff.

3. A receiving arrangement for a plurality of different frequencies, comprising a discriminator consisting of a frequency discriminating network and a pair of grid-controlled detector tubes, means normally biassing said tubes to plate current cutoff and for causing them to act as detectors of the square-law type, a first signal channel connected to the output of one tube, a separate signal channel connected to the output of the other tube, means to inject a local frequency simultaneously into both said tubes to produce a keyed tone in one channel when one of said tubes is plate current conductive and to produce a keyed tone in the other channel when the other of said tubes is plate current conductive both of said keyed tones being of the same frequency, and a signal transducer controlled by the keyed frequencies from both said channels.

4. A receiving arrangement for frequencyshifted carriers, comprising a frequency discriminator consisting of a frequency discriminating network and a pair of plural-grid electron tubes, means normally biassing the control grids of said tubes to plate current cutoff, means connecting the output of said discriminating network in divided balanced relation to the control grids of said tubes, means connecting a local source of injection frequency to another grid in each of said tubes, a separate output signal channel connected to the anodes respectively of said tubes, the control grids of said tubes being biassed to cause them to act as detector-s of the squarelaw type so that for a received carrier shift in one frequency direction from the mean carrier frequency only one of said tubes is plate current conductive and for a received carrier shift in the opposite direction from said mean carrier frequency only the other tube is plate current conductive.

5. A receiving arrangement according to claim 4, in which said local injection source has a frequency equal to the said mean carrier frequency, and each of said output channels includes a filter which only passes the frequency difference between said mean frequency and the frequencyshifted carrier.

6. A frequency shift receiving arrangement, comprising a current limiter device on which the detected frequency-shifted signals (FiAF) are impressed, a frequency discriminator consisting of a frequency discriminating network and a pair of plural-grid tubes, means to impress said detected signals on the input of said discriminator, a local source of injection oscillations having a frequency F, means to excite the control grids of said. tubes in push-pull relation by the signals from said discriminator network and means to excite another grid in each of said tubes by said local oscillations, means to bias the control grids of said tubes substantially to plate current cutoff and to cause each tube to act as a detector of the square-law type, the signals from said discriminator being applied to the control grids of said tubes to cause only one of said tubes to be plate current conductive when the received frequency shift signal deviates in one direction from the frequency F while the other tube only is plate current conductive when the received frequency shift signal deviates in the opposite direction from the frequency F, a filter connected in the output circuit of each tube to Select therefrom the same frequency AF, a pair of keying channels connected respectively to said output circuits, and a telegraph responder controlled by said keying channels.

'1. A frequency shift telegraph receiving arrangement, comprising a frequency discriminator consisting of a frequency discriminating network and a pair of plural-grid electron tubes, means normally biassing the control grids of said tubes to plate current cutoff, said tubes being connected to the output of said network in divided balanced relation so that for received frequency shift signals deviating in one direction from the mean frequency only one tube is plate current conductive and for frequency shift signals deviating in the opposite direction from the mean frequency only the other tube is plate current conductive, means to inject a local tone frequency into another grid in each of said tubes, each of said tubes having separate output circuits, a filter in one output circuit for biassing only said tone frequency, a filter in the other output circuit for biassing only the same tone frequency, separate keying channels connected to the outputs of said filters, and a telegraph device controlled by the outputs of said keyed channels.

8. A frequency shift receiving arrangement according to claim '7, in which each of said discriminator tubes is a pentode having its control grid excited by the signals from the frequency discriminating network, means connecting the screen grid of each tube for excitation by said local frequency source, the suppressor grid of each tube being connected substantially to cathode potential and the anode of each tube being connected to a corresponding one of said channels.

9. A receiving arrangement according to claim 3, in which said local frequency is in the audio frequency range.

GEORGE J. MAKI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Chapin Mar. 18, 1947 

